There is known a vehicular power transmitting system including an electrically controlled differential portion having a differential mechanism connected to a power transmitting path between an engine and a vehicle drive wheels, and a first electric motor which is operatively connected to the differential mechanism and an operating state of which is controlled to control a differential state of the differential mechanism, and further including a second electric motor operatively connected to the above-indicated drive wheels, and a transmission portion which constitutes a part of the power transmitting path. In this vehicular power transmitting system, the operating state of the differential mechanism is controlled with the operating state of the first electric motor being controlled, namely, with the first electric motor being switched to one of its different operating states including an electricity generating state, a drive force generating state and an idling state.
When the first electric motor is placed in its electricity generating state, an electric energy generated by the first electric motor is not only used to operate the second electric motor, but also is stored in a chargeable electric-energy storage device.
An amount of electric energy to be stored in the electric-energy storage device is controlled to prevent excessive charging and discharging of the electric-energy storage device, so as to keep the electric-energy storage device in a predetermined state of charge (SOC), for maintaining the performance of the electric-energy storage device. Accordingly, the amount of electric energy that can be stored in the electric-energy storage device is reduced (restricted) as the amount of electric energy stored in the electric-energy storage device approaches a predetermined upper limit.
For the reason described above, the electric energy generated by the above-indicated first electric motor may not be sufficiently consumed by the operation of the second electric motor and the charging of the electric-energy storage device. On the other hand, reduction of the amount of electric energy to be generated by the first electric motor may cause a change of the operating state of the above-indicated differential mechanism, resulting in a possibility of failure of the vehicular power transmitting system to provide a sufficient vehicle drive force.
Patent Document 1 discloses a technique to operate an optional device for consumption of the electric energy by the optional device, for preventing excessive charging of the electric-energy storage device when the amount of electric energy to be stored in the electric-energy storage device is restricted or limited while the vehicle is required to run with a drive force larger than usual, for instance, in an uphill running of the vehicle.    Patent Document 1: JP-2007-186005 A